1. Field of the Invention
This invention relates to a compact zoom lens system in which the lower limit of the back focal length is not restricted, such as a compact zoom lens system used for 35 mm lens-shutter type camera.
2. Description of the Relating Art
Conventionally, there is known a two components type compact zoom lens system which comprises from the object side to the image side, a first lens group of a negative refractive power and a second lens group of a positive refractive power. On the other hand, in order to increase the zoom ratio, for example, by U.S. Pat. No. 4,673,258, there are provided various types of zoom lens systems in which a lens group of a positive refractive power is located at the objective side end of the whole lens systems, and in which a plurality of lens groups are shifted in the zooming operation. Almost all types of such zoom lens systems, however, are designed for a single lens reflex camera which requires sufficient room for the reflex mirror to swing therein. Thus, since the lower limit of the back focal length is restricted the total length of such zoom lens system, i.e., the distance from the object side surface of the lens element located at the object side end to the image plane, is relatively long.
Contrarily, as a compact zoom lens system in which the lower limit of the back focal length is not restricted, there are shown a type of lens system which includes, from the object side to the image side, a first lens group of a positive refractive power and a second lens group of a negative refractive power. This type is disclosed in Japanese Laid-Open Patent Application No. Sho 56-128911, and Sho 57-201213. In this type, however, since the shifting amount of the lens group shifted in the zooming operation is relatively long, the lens barrel of such a zoom lens system becomes big. Furthermore, in this type, since an aperture stop is shifted along the optical axis in the zooming operation, the construction of the lens barrel becomes complicated. Especially, if this type of zoom lens system is used for a 35 mm lens-shutter camera, such a camera becomes big and complicated because it requires to locate a plurality of shutter blades and its driving unit near the aperture stop.
To solve these problems, U.S. Pat. No. 4,701,033, by the applicant for the present invention, proposes a four components type zoom lens system comprising, from the object side to the image side, a first lens unit of a positive refractive power, a second lens unit of a negative refractive power, a third lens unit of a positive refractive power, and a fourth lens unit of a negative refractive power. In this type, however, the zoom ratio of the whole lens system is small, i.e., 1.6X or thereabouts, so it is desired to magnify the zoom ratio.